Previous studies in this laboratory have demonstrated that during pregnancy there occurs an activation of a maternal spinal opioid system(s) that mediates the analgesia of pregnancy. One overall objective of this proposal is to characterize the specific opioid system(s) that is being modulated (type of opioid receptor and endogenous ligand that activates it) during gestation. The specific objectives are to (1) Determine the type(s) of spinal opiate receptor (mu. delta, kappa) that mediates the spinal opioid analgesia of pregnancy. (2) Determine the endogenous analgesia of pregnancy. (3) Determine the concentration of enkephalin, enkephalin precursors and preproenkephalin mRNA and the concentration of dynorphin and preprodynorphin mRNA in different regions of the spinal cord in pregnant (day 20) and non-pregnant animals. Additionally, the concentration profile of enkephalin, dynorphin, B-endorphin and total opioid activity in spinal cord superfusate will be determined and compared for pregnant (day 20) and non-pregnant rats. (4) Determine if pregnancy-induced analgesia is associated with modulation of spinal opiate receptors. The density and affinity of spinal mu, delta and kappa type of opiate receptors will be determine in pregnant rats (day 20) by radioligand binding techniques and compared with values that are obtained in the spinal cord of non- pregnant female rats.